Printhead assembly with similar substrate and nozzle guard material

ABSTRACT

A printhead assembly includes a substrate that defines a plurality of fluid inlet conduits. A plurality of nozzle assemblies is positioned on the substrate. Each nozzle assembly defines a nozzle chamber in fluid communication with a respective fluid inlet conduit and a nozzle opening from which fluid is ejected. A guard member is positioned on the substrate and is spaced from the nozzle assemblies. The guard member defines a plurality of apertures that are aligned with respective nozzle openings. The substrate and the guard member are of substantially the same material.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a Continuation of U.S. application Ser. No.10/778,089 filed on Feb. 17, 2004, which is a Continuation of U.S.application Ser. No. 10/052,449 filed on Jan. 23, 2002, now issued asU.S. Pat. No. 6,733,684.

FIELD OF THE INVENTION

The present invention relates to printheads. More particularly, thepresent invention relates to a printhead that incorporates nozzleassembly containment.

BACKGROUND TO THE INVENTION

Ink jet printers are a well-known and widely used form of printed mediaproduction. Ink is fed to an array of digitally controlled nozzles on aprinthead. As the print head passes over the media, ink is ejected fromthe array of nozzles to produce an image on the media.

Printer performance depends on factors such as operating cost, printquality, operating speed and ease of use. The mass, frequency andvelocity of individual ink drops ejected from the nozzles will affectthese performance parameters.

Recently, the array of nozzles has been formed usingmicro-electromechanical systems (MEMS) technology, which have mechanicalstructures with sub-micron thicknesses. This allows the production ofprintheads that can rapidly eject ink droplets sized in the picolitre(×10⁻¹² litre) range.

While the microscopic structures of these printheads can provide highspeeds and good print quality at relatively low costs, their size makesthe nozzles extremely fragile and vulnerable to damage from theslightest contact with fingers, dust or the media substrate. This canmake the printheads impractical for many applications where a certainlevel of robustness is necessary. Furthermore, a damaged nozzle may failto eject the ink being fed to it. As ink builds up and beads on theexterior of the nozzle, the ejection of ink from surrounding nozzle maybe affected and/or the damaged nozzle will simply leak ink onto theprinted substrate. Both situations are detrimental to print quality.

To address this, an apertured guard may be fitted over the nozzles toshield them against damaging contact. Ink ejected from the nozzlespasses through the apertures on to the paper or other substrate to beprinted. However, to effectively protect the nozzles, the apertures needto be as small as possible to maximize the restriction against theingress of foreign matter while still allowing the passage of the inkdroplets. Preferably, each nozzle would eject ink through its ownindividual aperture in the guard. However, given the microscopic scaleof MEMS devices, slight misaligniments between the guard and the nozzleswill obstruct the path of the ink droplets. Providing alignmentformations on the silicon wafer substrate for engaging complementaryformations on the guard can align the nozzles and respective aperturesto within 0.1 μm. However, while attaching the guard to the substrate,movement of the complementary formations into engagement with thealignment formations can damage the delicate nozzle structures.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided aprinthead that comprises

a substrate that defines a plurality of ink inlet conduits;

a plurality of nozzle assemblies that are positioned on the substrate,each nozzle assembly defining a nozzle chamber in fluid communicationwith a respective ink inlet conduit and a nozzle opening from which inkis ejected;

a guard member positioned on the substrate and spaced from the nozzleassemblies, the guard member defining a plurality of apertures that arealigned with respective nozzle openings; and

a plurality of containment walls disposed about respective nozzleassemblies and interposed between the substrate and the guard member todefine a plurality of containment chambers, at least one nozzle assemblybeing positioned in a respective containment chamber.

One nozzle assembly may be positioned in each containment chamber.

The substrate may incorporate drive circuitry. Each nozzle assembly mayinclude a micro-electromechanical ink ejection mechanism that isconnected to the drive circuitry to eject ink from the nozzle chamber onreceipt of a signal from the drive circuitry.

Each micro-electromechanical ink ejection mechanism may include athermal bend actuator that is connected to the drive circuitry to bedeflected as a result of differential thermal expansion and contractionon receipt of an electrical signal from the drive circuitry. An inkdisplacement member may be connected to the thermal bend actuator. Theink displacement member may be operatively arranged with respect to thenozzle chamber so that deflection of the thermal bend actuator resultsin the ejection of ink from the nozzle chamber.

Each ink displacement member may define part of the nozzle chamber suchthat movement of the ink displacement member under action of the thermalbend actuator results in a change in volume of the nozzle chamber andsubsequent ejection of ink from the nozzle chamber.

The substrate and the guard member may be of materials that have thesame coefficient of thermal expansion.

According to a second aspect of the invention there is provided a methodof fabricating a printhead for an ink jet printer, the printheadincluding:

a substrate carrying an array of nozzles for ejecting ink onto media tobe printed; and

an apertured nozzle guard to inhibit damaging contact with the nozzles,the method comprising the steps of:

forming the nozzles on the substrate using material etching anddeposition techniques such that the nozzles are reinforced bysacrificial material;

positioning the apertured nozzle guard over the exterior of the nozzlessuch that its apertures are in close registration with the nozzles; andsubsequently,

etching away the sacrificial material reinforcing the nozzles.

In this specification the term “nozzle” is to be understood as anelement defining an opening and not the opening itself.

In a preferred embodiment, alignment formations are formed on thesubstrate, the alignment formations being configured for engagement withcomplementary formations on the apertured nozzle guard; wherein,

engagement between the alignment formations and the complementaryformations holds the apertures in close registration with the nozzlessuch that the guard does not obstruct the normal trajectory of inkejected from the nozzles onto the media.

Preferably, etching plasma is injected through one or more of theapertures in the nozzle guard to release the sacrificial materialprotecting the nozzles, the released sacrificial material and etchingplasma flushing out through the apertures in the nozzle guard.

In one embodiment, the etching plasma is oxygen plasma and thesacrificial material is polyimide. In this embodiment, it is desirableto provide an inorganic seal between the alignment formation and thecomplementary formation.

The substrate may be a silicon wafer. The nozzle guard may have a shieldcontaining the apertures, the shield being spaced from the siliconsubstrate by integrally formed struts extending from the shield forengagement with the alignment formations. In one convenient form, thealignment formations are ridges on the silicon substrate positioned toengage the struts to maintain the apertures in alignment with the nozzlearray.

The alignment formations necessarily use up a proportion of the surfacearea of the printhead, and this adversely affects the nozzle packingdensity. The extra printhead chip area required for the same number ofnozzles will add to the cost of manufacturing the chip. However, whereassembling the printhead and the nozzle guard is not likely to besufficiently accurate, interengaging formations on the substrate and theguard will reduce the nozzle defect rate.

The nozzle guard may further include fluid inlet openings for directingfluid through the passages to inhibit the build up of foreign particleson the nozzle array. In this embodiment, the fluid inlet openings may bearranged in the struts.

It will be appreciated that, when air is directed through the openings,over the nozzle array and out through the passages, the build up offoreign particles on the nozzle array is inhibited.

The fluid inlet openings may be arranged in the support element remotefrom a bond pad of the nozzle array.

The present invention, ensures that the fragile MEMS nozzles areprotected during the manufacture and assembly of the printhead. Byproviding a nozzle guard for the printhead, the nozzle structures can beprotected from being touched or bumped against most other surfacesduring its operational life. To optimize the protection provided, theguard forms a flat shield covering the exterior side of the nozzleswherein the shield has an array of passages big enough to allow theejection of ink droplets but small enough to prevent inadvertent contactor the ingress of most dust particles. By forming the shield fromsilicon, its coefficient of thermal expansion substantially matches thatof the nozzle array. This will help to prevent the array of passages inthe shield from falling out of register with the nozzle array. Usingsilicon also allows the shield to be accurately micro-machined usingMEMS techniques. Furthermore, silicon is very strong and substantiallynon-deformable.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are now described, by way ofexample only, with reference to the accompanying drawings in which:

FIG. 1 shows a three dimensional, schematic view of a nozzle assemblyfor an ink jet printhead;

FIGS. 2 to 4 show a three dimensional, schematic illustration of anoperation of the nozzle assembly of FIG. 1;

FIG. 5 shows a three dimensional view of a nozzle array constituting anink jet printhead with a nozzle guard or containment walls;

FIG. 5 a shows a three-dimensional sectioned view of a printhead with anozzle guard and containment walls;

FIG. 5 b shows a sectioned plan view of nozzles taken through thecontainment walls isolating each nozzle;

FIG. 6 shows, on an enlarged scale, part of the array of FIG. 5;

FIG. 7 shows a three dimensional view of an ink jet printhead includinga nozzle guard without the containment walls;

FIG. 7 a shows an enlarged partial perspective of an ink jet print headwith alignment formations engaging the complementary formations on thenozzle guard;

FIG. 7 b shows a sectional view of a nozzle assembly encased insacrificial polyimide beneath the attached nozzle guard;

FIG. 7 c shows the etching plasma flow removing the sacrificial materialsurrounding the nozzle assembly;

FIGS. 8 a to 8 r show three-dimensional views of steps in themanufacture of a nozzle assembly of an ink jet printhead;

FIGS. 9 a to 9 r show sectional side views of the manufacturing steps;

FIGS. 10 a to 10 k show layouts of masks used in various steps in themanufacturing process;

FIGS. 11 a to 11 c show three dimensional views of an operation of thenozzle assembly manufactured according to the method of FIGS. 8 and 9;and

FIGS. 12 a to 12 c show sectional side views of an operation of thenozzle assembly manufactured according to the method of FIGS. 8 and 9.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring initially to FIG. 1 of the drawings, a nozzle assembly, inaccordance with the invention is designated generally by the referencenumeral 10. An ink jet printhead has a plurality of nozzle assemblies 10arranged in an array 14 (FIGS. 5 and 6) on a silicon substrate 16. Thearray 14 will be described in greater detail below.

The assembly 10 includes a silicon substrate or wafer 16 on which adielectric layer 18 is deposited. A CMOS passivation layer 20 isdeposited on the dielectric layer 18.

Each nozzle assembly 10 includes a nozzle 22 defining a nozzle opening24, a connecting member in the form of a lever arm 26 and an actuator28. The lever arm 26 connects the actuator 28 to the nozzle 22.

As shown in greater detail in FIGS. 2 to 4, the nozzle 22 comprises acrown portion 30 with a skirt portion 32 depending from the crownportion 30. The skirt portion 32 forms part of a peripheral wall of anozzle chamber 34. The nozzle opening 24 is in fluid communication withthe nozzle chamber 34. It is to be noted that the nozzle opening 24 issurrounded by a raised rim 36 which “pins” a meniscus 38 (FIG. 2) of abody of ink 40 in the nozzle chamber 34.

An ink inlet aperture 42 (shown most clearly in FIG. 6 of the drawings)is defined in a floor 46 of the nozzle chamber 34. The aperture 42 is influid communication with an ink inlet channel 48 defined through thesubstrate 16.

A wall portion 50 bounds the aperture 42 and extends upwardly from thefloor portion 46. The skirt portion 32, as indicated above, of thenozzle 22 defines a first part of a peripheral wall of the nozzlechamber 34 and the wall portion 50 defines a second part of theperipheral wall of the nozzle chamber 34.

The wall 50 has an inwardly directed lip 52 at its free end which servesas a fluidic seal which inhibits the escape of ink when the nozzle 22 isdisplaced, as will be described in greater detail below. It will beappreciated that, due to the viscosity of the ink 40 and the smalldimensions of the spacing between the lip 52 and the skirt portion 32,the inwardly directed lip 52 and surface tension function as aneffective seal for inhibiting the escape of ink from the nozzle chamber34.

The actuator 28 is a thermal bend actuator and is connected to an anchor54 extending upwardly from the substrate 16 or, more particularly fromthe CMOS passivation layer 20. The anchor 54 is mounted on conductivepads 56 which form an electrical connection with the actuator 28.

The actuator 28 comprises a first, active beam 58 arranged above asecond, passive beam 60. In a preferred embodiment, both beams 58 and 60are of, or include, a conductive ceramic material such as titaniumnitride (TiN).

Both beams 58 and 60 have their first ends anchored to the anchor 54 andtheir opposed ends connected to the arm 26. When a current is caused toflow through the active beam 58 thermal expansion of the beam 58results. As the passive beam 60, through which there is no current flow,does not expand at the same rate, a bending moment is created causingthe arm 26 and, hence, the nozzle 22 to be displaced downwardly towardsthe substrate 16 as shown in FIG. 3. This causes an ejection of inkthrough the nozzle opening 24 as shown at 62. When the source of heat isremoved from the active beam 58, i.e. by stopping current flow, thenozzle 22 returns to its quiescent position as shown in FIG. 4. When thenozzle 22 returns to its quiescent position, an ink droplet 64 is formedas a result of the breaking of an ink droplet neck as illustrated at 66in FIG. 4. The ink droplet 64 then travels on to the print media such asa sheet of paper. As a result of the formation of the ink droplet 64, a“negative” meniscus is formed as shown at 68 in FIG. 4 of the drawings.This “negative” meniscus 68 results in an inflow of ink 40 into thenozzle chamber 34 such that a new meniscus 38 (FIG. 2) is formed inreadiness for the next ink drop ejection from the nozzle assembly 10.

Referring now to FIGS. 5 and 6 of the drawings, the nozzle array 14 isdescribed in greater detail. The array 14 is for a four-color printhead.Accordingly, the array 14 includes four groups 70 of nozzle assemblies,one for each color. Each group 70 has its nozzle assemblies 10 arrangedin two rows 72 and 74. One of the groups 70 is shown in greater detailin FIG. 6.

To facilitate close packing of the nozzle assemblies 10 in the rows 72and 74, the nozzle assemblies 10 in the row 74 are offset or staggeredwith respect to the nozzle assemblies 10 in the row 72. Also, the nozzleassemblies 10 in the row 72 are spaced apart sufficiently far from eachother to enable the lever arms 26 of the nozzle assemblies 10 in the row74 to pass between adjacent nozzles 22 of the assemblies 10 in the row72. It is to be noted that each nozzle assembly 10 is substantiallydumbbell shaped so that the nozzles 22 in the row 72 nest between thenozzles 22 and the actuators 28 of adjacent nozzle assemblies 10 in therow 74.

Further, to facilitate close packing of the nozzles 22 in the rows 72and 74, each nozzle 22 is substantially hexagonally shaped.

It will be appreciated by those skilled in the art that, when thenozzles 22 are displaced towards the substrate 16, in use, due to thenozzle opening 24 being at a slight angle with respect to the nozzlechamber 34 ink is ejected slightly off the perpendicular. It is anadvantage of the arrangement shown in FIGS. 5 and 6 of the drawings thatthe actuators 28 of the nozzle assemblies 10 in the rows 72 and 74extend in the same direction to one side of the rows 72 and 74. Hence,the ink ejected from the nozzles 22 in the row 72 and the ink ejectedfrom the nozzles 22 in the row 74 are offset with respect to each otherby the same angle resulting in an improved print quality.

Also, as shown in FIG. 5 of the drawings, the substrate 16 has bond pads76 arranged thereon which provide the electrical connections, via thepads 56, to the actuators 28 of the nozzle assemblies 10. Theseelectrical connections are formed via the CMOS layer (not shown).

Referring to FIGS. 5 a and 5 b, the nozzle array 14 shown in FIG. 5 hasbeen spaced to accommodate a containment formation surrounding eachnozzle assembly 10. The containment formation is a containment wall 144surrounding the nozzle 22 and extending from the silicon substrate 16 tothe underside of an apertured nozzle guard 80 to form a containmentchamber 146. If ink is not properly ejected because of nozzle damage,the leakage is confined so as not to affect the function of surroundingnozzles. It is also envisaged that each containment chamber 146 willhave the ability to detect the presence of leaked ink and providefeedback to the microprocessor controlling the actuation of the nozzlearray 14. Using a fault tolerance facility, the damaged can becompensated for by the remaining nozzles in the array 14 therebymaintaining print quality.

The containment walls 144 necessarily occupy a proportion of the siliconsubstrate 16 which decreases the nozzle packing density of the array.This in turn increases the production costs of the printhead chip.However where the manufacturing techniques result in a relatively highnozzle attrition rate, individual nozzle containment formations willavoid, or at least minimize any adverse effects to the print quality.

It will be appreciated by those in the art, that the containmentformation could also be configured to isolate groups of nozzles.Isolating groups of nozzles provides a better nozzle packing density butcompensating for damaged nozzles using the surrounding nozzle groups ismore difficult.

Referring to FIG. 7, a nozzle guard for protecting the nozzle array isshown. With reference to the previous drawings, like reference numeralsrefer to like parts, unless otherwise specified.

A nozzle guard 80 is mounted on the silicon substrate 16 of the array14. The nozzle guard 80 includes a shield 82 having a plurality ofapertures 84 defined therethrough. The apertures 84 are in registrationwith the nozzle openings 24 of the nozzle assemblies 10 of the array 14such that, when ink is ejected from any one of the nozzle openings 24,the ink passes through the associated passage before striking the printmedia.

The guard 80 is silicon so that it has the necessary strength andrigidity to protect the nozzle array 14 from damaging contact withpaper, dust or the users' fingers. By forming the guard from silicon,its coefficient of thermal expansion substantially matches that of thenozzle array. This aims to prevent the apertures 84 in the shield 82from falling out of register with the nozzle array 14 as the printheadheats up to its normal operating temperature. Silicon is also wellsuited to accurate micro-machining using MEMS techniques discussed ingreater detail below in relation to the manufacture of the nozzleassemblies 10.

The shield 82 is mounted in spaced relationship relative to the nozzleassemblies 10 by limbs or struts 86. One of the struts 86 has air inletopenings 88 defined therein.

In use, when the array 14 is in operation, air is charged through theinlet openings 88 to be forced through the apertures 84 together withink traveling through the apertures 84.

The ink is not entrained in the air as the air is charged through theapertures 84 at a different velocity from that of the ink droplets 64.For example, the ink droplets 64 are ejected from the nozzles 22 at avelocity of approximately 3 m/s. The air is charged through theapertures 84 at a velocity of approximately 1 m/s.

The purpose of the air is to maintain the apertures 84 clear of foreignparticles. A danger exists that these foreign particles, such as dustparticles, could fall onto the nozzle assemblies 10 adversely affectingtheir operation. With the provision of the air inlet openings 88 in thenozzle guard 80 this problem is, to a large extent, obviated.

The alignment between the apertures 84 and the nozzles 22 is crucial.However, the microscopic scale of MEMS devices makes precise positioningof the guard 80 over the nozzles difficult. As shown in FIG. 7 a, thesilicon wafer or substrate 16 can be provided with alignment formationssuch as ridges 148 for engaging the free ends of the struts 86. If thesacrificial material used is polyimide, an inorganic seal 150 issandwiched between the end of the support 86 and the ridge 148. Theridges 148 may be accurately formed together with the nozzles 22 usingthe same etching and deposition techniques.

FIG. 7 a shows trapped sacrificial material such as polyimide formingthe alignment ridges 148. In other arrangements, extra ridges 148 engagethe containment walls 144 shown in FIGS. 5 a and 5 b. In this form, theridges 148 will occupy some surface area and adversely affect the nozzlepacking density, but it will firmly hold each aperture 84 in alignmentwith the respective nozzles 22. An inorganic seal 150 is positionedbetween the struts 86 and the ridges 148.

Of course other arrangements can provide alignment formations such asrecesses or sockets in the wafer substrate 16 that engage complementaryformations provided on the guard 80.

Alignment formations formed using CMOS etching and deposition techniquescan provide an alignment accuracy of the order of 0.1 μm.

While aligning the guard 80 on the alignment formations 148, the fragilenozzles 22 are prone to damage because of contact with the struts 86. Asshown in FIG. 7 b, the nozzles 22 can be encased in sacrificial material152 for protection until the guard 80 has been aligned and fixed to theridges 148. Referring the FIG. 7 c, once the guard 80 is in place, theprotective sacrificial material 152 is released by an oxygen plasma etch154 and removed through the apertures 84.

Referring now to FIGS. 8 to 10 of the drawings, a process formanufacturing the nozzle assemblies 10 is described.

Starting with the silicon substrate 16, the dielectric layer 18 isdeposited on a surface of the wafer 16. The dielectric layer 18 is inthe form of approximately 1.5 microns of CVD oxide. Resist is spun on tothe layer 18 and the layer 18 is exposed to mask 100 and is subsequentlydeveloped.

After being developed, the layer 18 is plasma etched down to the siliconlayer 16. The resist is then stripped and the layer 18 is cleaned. Thisstep defines the ink inlet aperture 42.

In FIG. 8 b of the drawings, approximately 0.8 microns of aluminum 102is deposited on the layer 18. Resist is spun on and the aluminum 102 isexposed to mask 104 and developed. The aluminum 102 is plasma etcheddown to the oxide layer 18, the resist is stripped and the device iscleaned. This step provides the bond pads and interconnects to theinkjet actuator 28. This interconnect is to an NMOS drive transistor anda power plane with connections made in the CMOS layer (not shown).

Approximately 0.5 microns of PECVD nitride is deposited as the CMOSpassivation layer 20. Resist is spun on and the layer 20 is exposed tomask 106 whereafter it is developed. After development, the nitride isplasma etched down to the aluminum layer 102 and the silicon layer 16 inthe region of the inlet aperture 42. The resist is stripped and thedevice cleaned.

A layer 108 of a sacrificial material is spun on to the layer 20. Thelayer 108 is 6 microns of photosensitive polyimide or approximately 4 μmof high temperature resist. The layer 108 is softbaked and is thenexposed to mask 110 whereafter it is developed. The layer 108 is thenhardbaked at 400° C. for one hour where the layer 108 is comprised ofpolyimide or at greater than 300° C. where the layer 108 is hightemperature resist. It is to be noted in the drawings that thepattern-dependent distortion of the polyimide layer 108 caused byshrinkage is taken into account in the design of the mask 110.

In the next step, shown in FIG. 8 e of the drawings, a secondsacrificial layer 112 is applied. The layer 112 is either 2 μm ofphotosensitive polyimide which is spun on or approximately 1.3 μm ofhigh temperature resist. The layer 112 is softbaked and exposed to mask114. After exposure to the mask 114, the layer 112 is developed. In thecase of the layer 112 being polyimide, the layer 112 is hardbaked at400° C. for approximately one hour. Where the layer 112 is resist, it ishardbaked at greater than 300° C. for approximately one hour.

A 0.2-micron multi-layer metal layer 116 is then deposited. Part of thislayer 116 forms the passive beam 60 of the actuator 28.

The layer 116 is formed by sputtering 1,000 Å of titanium nitride (TiN)at around 300° C. followed by sputtering 50 Å of tantalum nitride (TaN).A further 1,000 Å of TiN is sputtered on followed by 50 Å of TaN and afurther 1,000 Å of TiN. Other materials which can be used instead of TiNare TiB₂, MoSi₂ or (Ti, Al)N.

The layer 116 is then exposed to mask 118, developed and plasma etcheddown to the layer 112 whereafter resist, applied for the layer 116, iswet stripped taking care not to remove the cured layers 108 or 112.

A third sacrificial layer 120 is applied by spinning on 4 μm ofphotosensitive polyimide or approximately 2.6 μm high temperatureresist. The layer 120 is softbaked whereafter it is exposed to mask 122.The exposed layer is then developed followed by hard baking. In the caseof polyimide, the layer 120 is hardbaked at 400° C. for approximatelyone hour or at greater than 300° C. where the layer 120 comprisesresist.

A second multi-layer metal layer 124 is applied to the layer 120. Theconstituents of the layer 124 are the same as the layer 116 and areapplied in the same manner. It will be appreciated that both layers 116and 124 are electrically conductive layers.

The layer 124 is exposed to mask 126 and is then developed. The layer124 is plasma etched down to the polyimide or resist layer 120whereafter resist applied for the layer 124 is wet stripped taking carenot to remove the cured layers 108, 112 or 120. It will be noted thatthe remaining part of the layer 124 defines the active beam 58 of theactuator 28.

A fourth sacrificial layer 128 is applied by spinning on 4 μm ofphotosensitive polyimide or approximately 2.6 μm of high temperatureresist. The layer 128 is softbaked, exposed to the mask 130 and is thendeveloped to leave the island portions as shown in FIG. 9 k of thedrawings. The remaining portions of the layer 128 are hardbaked at 400°C. for approximately one hour in the case of polyimide or at greaterthan 300° C. for resist.

As shown in FIG. 8 l of the drawing a high Young's modulus dielectriclayer 132 is deposited. The layer 132 is constituted by approximately 1μm of silicon nitride or aluminum oxide. The layer 132 is deposited at atemperature below the hardbaked temperature of the sacrificial layers108, 112, 120, 128. The primary characteristics required for thisdielectric layer 132 are a high elastic modulus, chemical inertness andgood adhesion to TiN.

A fifth sacrificial layer 134 is applied by spinning on 2 μm ofphotosensitive polyimide or approximately 1.3 μm of high temperatureresist. The layer 134 is softbaked, exposed to mask 136 and developed.The remaining portion of the layer 134 is then hardbaked at 400° C. forone hour in the case of the polyimide or at greater than 300° C. for theresist.

The dielectric layer 132 is plasma etched down to the sacrificial layer128 taking care not to remove any of the sacrificial layer 134.

This step defines the nozzle opening 24, the lever arm 26 and the anchor54 of the nozzle assembly 10.

A high Young's modulus dielectric layer 138 is deposited. This layer 138is formed by depositing 0.2 μm of silicon nitride or aluminum nitride ata temperature below the hardbaked temperature of the sacrificial layers108, 112, 120 and 128.

Then, as shown in FIG. 8 p of the drawings, the layer 138 isanisotropically plasma etched to a depth of 0.35 microns. This etch isintended to clear the dielectric from the entire surface except the sidewalls of the dielectric layer 132 and the sacrificial layer 134. Thisstep creates the nozzle rim 36 around the nozzle opening 24 which “pins”the meniscus of ink, as described above.

An ultraviolet (UV) release tape 140 is applied. 4 μm of resist is spunon to a rear of the silicon wafer substrate 16. The wafer substrate 16is exposed to mask 142 to back etch the wafer substrate 16 to define theink inlet channel 48. The resist is then stripped from the wafer 16.

A further UV release tape (not shown) is applied to a rear of the wafersubstrate 16 and the tape 140 is removed. The sacrificial layers 108,112, 120, 128 and 134 are stripped in oxygen plasma to provide the finalnozzle assembly 10 as shown in FIGS. 8 r and 9 r of the drawings. Forease of reference, the reference numerals illustrated in these twodrawings are the same as those in FIG. 1 of the drawings to indicate therelevant parts of the nozzle assembly 10. FIGS. 11 and 12 show theoperation of the nozzle assembly 10, manufactured in accordance with theprocess described above with reference to FIGS. 8 and 9 and thesefigures correspond to FIGS. 2 to 4 of the drawings.

It will be appreciated by persons skilled in the art that numerousvariations and/or modifications may be made to the invention as shown inthe specific embodiments without departing from the spirit or scope ofthe invention as broadly described. The present embodiments are,therefore, to be considered in all respects as illustrative and notrestrictive.

1. A printhead assembly that comprises a substrate defining a pluralityof ink inlets; a plurality of nozzle assemblies positioned on thesubstrate, each nozzle assembly defining a nozzle chamber in fluidcommunication with a respective ink inlet and a nozzle opening fromwhich ink is ejected; and a guard member positioned on the substrate andspaced from the nozzle assemblies, the guard member defining a pluralityof apertures that are aligned with respective nozzle openings, whereinthe substrate and the guard member are of substantially the samematerial.
 2. A printhead assembly as claimed in claim 1, in which boththe substrate and the guard member are substantially of silicon.
 3. Aprinthead assembly as claimed in claim 1, in which a plurality ofcontainment walls are interposed between the substrate and the guardmember to define a plurality of containment chambers, at least onenozzle assembly being positioned in each containment chamber, thecontainment walls being of substantially the same material as the guardmember and the substrate.
 4. A printhead assembly as claimed in claim 1,in which one nozzle assembly is positioned in each containment chamber.5. A printhead assembly as claimed in claim 1, in which the substrateincorporates drive circuitry and each nozzle assembly includes amicro-electromechanical ink ejection mechanism that is connected to thedrive circuitry to eject ink from the nozzle chamber on receipt of asignal from the drive circuitry.
 6. A printhead assembly as claimed inclaim 5, in which each micro-electromechanical ink ejection mechanismincludes a thermal bend actuator that is connected to the drivecircuitry to be deflected as a result of differential thermal expansionand contraction on receipt of an electrical signal from the drivecircuitry, and an ink displacement member that is connected to thethermal bend actuator, the ink displacement member being operativelyarranged with respect to the nozzle chamber so that deflection of thethermal bend actuator results in the ejection of ink from the nozzlechamber.
 7. A printhead assembly as claimed in claim 6, in which eachink displacement member defines part of the nozzle chamber such thatmovement of the ink displacement member under action of the thermal bendactuator results in a change in volume of the nozzle chamber andsubsequent ejection of ink from the nozzle chamber.
 8. A printer thatincludes a printhead assembly as claimed in claim 1.